Rim sprockets can be described as having a star shaped center section (i.e., having radially extended teeth) positioned between disc shaped side walls. The spacings between the teeth circumferentially, and between the side walls laterally, define gullets that receive the drive tangs of center links of a saw chain, and the side walls further define outer peripheral support surfaces or rails which support the side links of the saw chain. The sprockets have a spline shaped center opening through the thickness of all three sections that receive a drive shaft, e.g. of an adapter driven by the chainsaw's engine. In an example of such a chainsaw, a centrifugal clutch driven by the engine engages the cup and rotates the adapter shaft to rotatively drive the rim sprocket and thus the saw chain mounted on the rim sprocket. The saw chain is thereby driven around a guide bar of the chainsaw for cutting trees or logs and the like.
The drive sprocket is a key component of the chainsaw's drive system and is subject to harsh abuse and rapid wearing. It is desired that the sprocket be made to withstand the severe abuse over many hours of use, e.g. the lifetime of several cutting chains and yet be produced as inexpensively as feasible.
A process has been developed for making such sprockets. A mold tree is formed. The mold tree is a plastic form with a vertical center section made of many interconnected segments having spokes radiating horizontally. Secured to each spoke is a sprocket mold or mold form, that is in the shape of the rim sprocket to be produced. This tree is coated with ceramic and the plastic form is burned away leaving a ceramic mold. Passageways are thereby provided down the center of the mold tree created by the burned out center section (referred to as a sprue) and through the burned out spokes (referred to as gates) and into each sprocket mold cavity. Molten steel is poured through the passageway and into the numerous sprocket mold cavities in a single operation. When cooled, the ceramic mold surrounding the solidified sprockets is removed but, notwithstanding, the sprockets remain interconnected via the steel that has hardened in the gates. The hardened steel formed in the gates is sometimes referred to as stems. As designed, the steel of the stem formed inside the gate and which is connected to the sprocket is small in dimension and the sprocket can be broken away from the hardened metal formed in the sprue. Any nubbin of the stem remaining on the sprocket can be readily ground away to remove any sign of the interconnection, and thus rendering the sprocket ready for final processing e.g. heat treating.
The process as described has a number of critical aspects and as a result there are problems that are herein addressed. The molten steel is desirably poured when at an established molten temperature to ensure complete filling of the mold forms and to ensure a desired steel composition of the end product. The stems generated at the gates should be configured so as to permit a clean break away of the solidified sprockets. The metal throughout the sprocket form is preferably uniformly dense, i.e., devoid of porosity. Other desirable features for the rim sprocket are that the sprockets as produced facilitate wood chip removal during a wood cutting operation, and that the weight of the sprocket be minimized.